sum1 help this is really getiin on my nerves

im having a problem with my amp well not a problem just im wandering whether its s*** or not i bought it for 60 qwid its a large 50 watt marshall g50r cd and its all good apart from the palm muting....i hear bands and when they palm mute its ot a real like erm how do i put it like a growl to it its actually got a tone...like on the tab on here for number of the beast it says the hole intro to it iis palm muted it sounds kickass from iron maiden when i play it....its like a dull thud with a slight note....

it could be my guitar its a les paul stagg if that makes any diffrence thanks for any help....

and please no abuse i admit i dont kno much but at least im askin and im not preaching some bull s***